You put one of the notches over the wire, and work the knife left and right bending the wire at the notch.
Same as bending wire back and forth in your fingers, it thins, heats and eventually breaks.
Largely useless IMO too I must say. If you're working a property with fencing you'll have cutters. If you're out in the wilderness there aren't going to be fences.
On the odd chance you come across something it'd be quicker to jump over or go through it anyway. Who'd spend the time cutting one, really?
Most of these things IMO are to fill some special forces type fantasy. I don't have to silently infiltrate a rebel base very often so I don't see a need for myself